This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. FPOP originally uses fast photolysis to produce radicals for footprinting proteins and their complexes. The original reagent we developed for FPOP was OH radical produced by photolysis of hydrogen peroxide with an excimer laser. In this project we are exploring other sources to produce radicals with different reactivity and selectivity, and even different modifications. Radical under investigation include carbonate radical anion, iodine radical. We are mapping out the reactivities and selectivities of these radicals and looking for other radical species to use in FPOP. This constitutes one of the major advantages of FPOP over other forms of oxidative modification strategies, and this is, FPOP can accommodate a variety of reactive reagents. Our goal is to build a "tool kit" of reagents for protein footprinting by reactive reagents.